The present invention refers to a process for treatment of liquid and solid effluents allowing recovering metals through precipitation of co-sulphide, enabling magnesium recovery and providing sulphate recovery as gypsum and ammonium sulphate.
The main objectives of any method for treatment of liquid and solid effluent are (i) neutralize and fit them in a proper manner to be launched and returned to environment and (ii) obtain recyclable byproducts to be reused in the manner of a captive consumption and/or via commercialization to third parties.
In utilization of lateritic and nickeliferous ores, several factors affect nickel production process by hydrometallurgical route and high pressure acid leaching (HPAL), namely:                Geological origin of beds;        Ore mineralogical composition;        Ore granulometric distribution;        Processing operation conditions;        Configuration of ore preparation, leaching, precipitation, solvent extraction and electro-refining systems.        
High pressure acid leaching (HPAL) is recommended for predominantly limonitic ores presenting low contents of magnesium—usually limited to 4% at most—once ores bearing high contents of magnesium present a high consumption of sulphuric acid.
Processes used to produce nickel by known routes, such as HPAL, as illustrated in FIG. 1, essentially comprises the following steps: (i) preparation of nickel lateritic ore, (ii) nickel leaching under pressure with sulphuric acid, (iii) nickel precipitation, (iv) re-leaching, (v) nickel solvent-extraction and (vi) electro-refining for production of cathode nickel (metallic nickel with 99.95% purity). In view of the significant presence of cobalt in ore, the same is will be obtained as a coproduct, also in metallic form.
This technology is the most indicated process to extract nickel and cobalt from limonitic laterites due to the following advantages:                limonitic laterites present low magnesium contents and, therefore, a low consumption of sulphuric acid;        Low operating cost due to low cost of sulphuric acid and low specific consumption thereof;        No drying or reduction stage is required, as raw laterite (Run Of Mine—ROM) is used in suspension form;        High selectivity to relevant metals;        Sulphur dioxide emissions below environmental standards;        Recovery rates above 90% for nickel and cobalt contained in the ore.        
Leaching under pressure is usually performed in titanium-coated autoclaves, in temperatures ranging from 463 to 518° F. (245 to 270° C.). In this process, autoclave feed suspension contains approximately 40-45% solids, when previously heated with steam. In some ores, due to nature of argillaceous ore which are present, this concentration may be limited up to 25% to 30%. Thickening level of the suspension has a significant influence in autoclave capacity, which constitutes an equipment with a high capital cost. Leaching mechanism involves acid dissolution in high temperature of nickel and cobalt contained in the host ore matrix. In these conditions, iron dissolution of these ores occur, followed by formation of sulphates, which in high temperature conditions react with water to form hematite, thereby regenerating sulphuric acid:2FeOOH+H2SO4=Fe2(SO4)3+4H2OFe2Si4O10(OH)2+3H2SO4=Fe2(SO4)3+4SiO2+4H2OFe2(SO4)3+2H2O=2Fe(OH)SO4+H2SO4 2Fe(OH)SO4+H2O=Fe2O3+2H2SO4 
Extraction levels of this process reach values from 92% to 96% for nickel and 90 to 92% for cobalt. Normally, in order to obtain this degree of extraction, reaction suspension after chemical attack should present a residual free acid concentration between 30 and 50 g/l.
After leaching, autoclave-derived suspension is depressurized and cooled in expansion chambers (flash vessels), at approximately 212° F. (100° C.), the remaining solids being separated from liquid phase. Solid-liquid separation is performed in decanters operating in countercurrent (CCD), generating a liquor laden with nickel, cobalt, magnesium, manganese, zinc, copper, iron and other metals sulphates. Then, nickel and cobalt present in the liquor are precipitated in the manner of sulphides (by using H2S), carbonates (by using ammonium carbonate) or as hydroxides, by using magnesia—MgO. These intermediate products usually present contents of 55% (Ni+Co) for sulphides (MSP—Mixed Sulphide Precipitate) and 40 to 45% (Ni+Co) in case of hydroxides (MHP—Mixed Hydroxide Precipitate). Recovery of these metals is also possible through solvent extraction directly applied to liquor derived from decantation system. It should be noted that MSP process enables to obtain a product with a higher contents of valuable metals and a lower level of contamination by manganese, magnesium and sulphates. However, production by MSP route involves a high capital cost for auxiliary facilities, once hydrogen and hydrogen sulphite units are necessary, requiring sophisticated security and handling systems for these products.
In the next step—refining—intermediate products (sulphites or nickel and cobalt hydroxide) are re-leached and so, dissolved, they are submitted to purification treatments, such as (i) solvent extraction for nickel and cobalt separation and (ii) electrolysis (electro-refining) aiming to reach higher degrees of purity.
In nickel production process by HPAL route from ore containing silicate and magnesium carbonate, generation of liquid effluent in proportions from 250 to 400 m3/kg of Ni produced takes place, containing essentially magnesium and sulphate, and in a small quantity, cobalt, zinc, manganese, nickel, iron, chromium, among other elements. Table 1 presents chemical composition of the effluent to be treated and shows the significant sulphate and magnesium contents.
TABLE 1Composition of effluent to be treatedComponentsUnit.ContentsNippm2.00Coppm6.00Znppm0.90Mnppm40.00Feppm10.00Crg/l3.45Mgg/l18.00SO4g/l75.00NH3g/L0.50pH7.5
Effluents treatment process has being exhaustively used to avoid sending-off noxious waste in the environment. Most part of the final product of the effluents treatment has being discharged when it could be reused in the main process or for any other situation in many field. In this sense, U.S. Pat. No. 6,861,041 refers to a method for treating and upgrading effluents containing at least a metallic sulphate. This method comprises steps which consist in: adding at least a base to the effluent to precipitate the metallic ions in the form of metallic hydroxides Mea(OH)e; separating the precipitated hydroxide calcium sulphate CaSO4; and separating the calcium sulphate CaSO4 precipitated during the preceding step. Said method enables to obtain calcium sulphate (white gypsum) substantially free of metals and therefore capable of being upgrade and metallic oxy-hydroxides likewise capable of being upgraded. Even though this process allows removing metallic sulphate from the effluent, the final product of the treatment, the sulphate, is not reused.
Further, GB patent 1346524 discloses a treatment of polluted water that comprises mixing the water with one or more of sulphuric acid, nitric acid and hydrochloric acid to lower the pH of the water to at least 2.5, maintaining this pH for a time sufficient to destroy bacteria present, then bringing the mixture into contact with an acid soluble form of iron for a time sufficient for the iron to react with part of the acid and then raising the pH of the mixture to 7 by the addition of an alkaline solution. During the contact with the acid soluble form of iron, at least one salt selected from the sulphates and nitrates of iron and aluminium, may be present. The mixture of acid and water is preferably contacted with at least 3.2 sq. ft. (0.3 m2) of iron per gallon per minute of acid and water mixture; Ammonium, sodium and potassium hydroxides are suitable alkaline solutions for use in the neutralization step.